One type of film-encased electric device is a film-encased cell. Heretofore, there has been known a film-encased cell including a cell element sandwiched by encasing films placed on respective opposite sides in the thickness direction thereof and having dimensions greater than the planar dimensions of the cell element, the cell element being hermetically sealed (also simply referred to as “sealed”) by the opposite encasing films joined to each other around the cell element. Positive and negative poles tabs are connected as electrodes to the cell element. With the cell element sealed, the tabs extend through the encasing films. Each of the encasing films generally comprises a laminated film comprising a metal layer and a heat-sealable resin layer. The cell element is sealed by the heat-sealable resin layers which are thermally fused to each other.
As with metal cans or the like which employ encasing materials other than film, cells using an encasing film material are required for thermally fused regions to be sealed reliably in order to prevent ambient air from entering the cell and also to prevent the electrolytic solution from leaking out. In particular, sealing reliability is important for cells containing a nonaqueous electrolyte (hereinafter referred to as “nonaqueous electrolyte cell”). If a heat-seal failure occurs, the electrolyte will deteriorate due to exposure to components of the ambient air, resulting in a significant reduction in cell performance.
If a voltage out of the rated range of a cell is applied to the cell while the cell is in use, then gas may be produced due to the electrolysis of the electrolyte solvent. Furthermore, if the cell is used at a high temperature outside of the rated range, then a substance is produced which is responsible for gas due to the decomposition of the electrolyte salt. Basically, it is ideal to use the cell in the rated range so as not to produce gas from the cell. However, an abnormal voltage may possibly be applied in the event of a failure of a control circuit of the cell for some reason, or a high ambient temperature may occur for some reasons, tending to cause the cell to produce a large amount of gas.
The generation of a gas within a cell results in an internal pressure buildup in the cell. In order to prevent the cell from suffering an accidental explosion owing to an extremely high internal pressure buildup, many cells that use a metal can as an encasing member have a pressure safety valve for releasing gas from the cell in the event of an internal pressure buildup in the cell. It is difficult for film-encased cells that use a film as an encasing material to have a pressure safety valve because of structural limitations. If a film-encased cell undergoes internal pressure buildup, then the film will expanded until finally it is ruptured, allowing a gas to be ejected through the rupture. It is impossible to identify a location where such a film rupture will occur. Depending on the location of a film rupture, the cell may possibly adversely affect devices or members in the surrounding area.
There have been proposed some pressure releasing structures for eliminating trouble due to the generation of gas in conventional film-encased cells.
For example, as shown in FIG. 1, JP-A No. 2004-55290 discloses film-encased cell 101 having encasing films 104 that is heat-sealed around a cell element (not shown) to form heat-sealed area 106 including a portion projecting toward a region wherein the cell element is housed. Encasing films 104 have gas releaser 107 with its tip end disposed in the projecting portion of heat-sealed area 106. Gas releaser 107 is formed, but not by heat-sealing process.
When encasing films 104 are expanded by an internal pressure buildup caused by a gas produced in film-encased cell 101, heat-sealed area 106 is subjected to a peel-off stress. Since heat-sealed area 106 includes the projecting portion, the peel-off stress concentrates on the projecting portion of heat-sealed area 106, causing encasing films 104 to peel off progressively more especially in the projecting portion than in other portions thereof. When the peeling of encasing films 104 reaches gas releaser 107, the interior of film-encased cell 101 comes into contact with the ambient air, and gas releaser 107 releases the gas.